1. Field of the Invention
The present invention relates to a control system for vehicles, which calculates a desired driving force required by the vehicle depending on operating conditions of the vehicle, and controls the driving force of the vehicle to the calculated desired driving force.
2. Description of the Related Art
Conventionally, there is known a throttle valve control system which electrically controls the opening of a throttle valve of an internal combustion engine installed in an automotive vehicle, according to a stepping-on amount of an accelerator pedal of the vehicle, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 63-268943. According to the known control system, when the vehicle is traveling on a street congested with traffic, the gain or change rate of the valve opening of the throttle valve relative to the stepping-on amount of the accelerator pedal is decreased below a normal value so as to improve the drivability during traveling on the congested street.
The known control system, however, does not contemplate the time period over which the vehicle stands in an idle state on a congested street. As a result, if the vehicle stands over a long time period, there occurs a change in the driver's sense of the behavior of the vehicle in response to operation of the accelerator pedal so that the driver feels degraded drivability or insufficient accelerability when he starts the vehicle.
Further, when the vehicle is cruising or traveling at a nearly constant vehicle speed over a long time period, the driver's sense changes so that he feels degraded drivability when he accelerates the vehicle from the cruising state.
On the other hand, there has been conventionally known a control system for vehicles from Japanese Laid-Open Patent Publication (Kokai) No. 3-163256, which calculates by the use of fuzzy inference a desired driving force of the vehicle according to the stepping-on amount of the accelerator pedal and the vehicle speed, and controls the gear position of an automatic transmission of the vehicle and the valve opening of a throttle valve of an internal combustion engine which drives the vehicle so that the driving force of the vehicle becomes equal to the calculated desired driving force.
This known control system, however, changes the gear position of the automatic transmission according to the desired driving force, and as a result, the gear position is frequently changed (so-called shift-busy state), which can degrade the drivability.
On the other hand, in an automotive vehicle with an internal combustion engine with an automatic transmission installed therein, when the vehicle is in a so-called stall state, i.e., an operating condition in which the shift position of the automatic transmission is selected to a traveling position other than a neutral (N) position or a parking (P) position, the vehicle speed is zero, the vehicle is braked, and at the same time the engine rotational speed is higher than a predetermined speed with the accelerator pedal stepped on, the rotational speed ratio between the output shaft and input shaft of the automatic transmission is zero (the output shaft rotational speed is zero) with the difference in rotational speed between the output and input shafts being absorbed by a torque converter of the automatic transmission. On this occasion, almost the entire energy from the engine is converted to heat generated by the torque converter.
If such a stall state lasts over a long time period, component parts of the automatic transmission can be damaged by heat radiated from the torque converter. To protect the automatic transmission, therefore, conventionally the output torque from the engine is reduced to decrease the engine rotational speed by means of fuel cut (interruption of the fuel supply to the engine) or retarding the ignition timing to thereby reduce the amount of heat generated by the torque converter.
However, if fuel cut is employed to reduce the engine output torque for protection of the automatic transmission when the vehicle is in a stall state, exhaust gases from the engine become lean so that the exhaust gas temperature abnormally rises, which can damage a catalytic converter arranged in the exhaust pipe. Further, if the ignition timing is retarded, there can occur irregular combustion within the engine so that unburnt fuel gas is emitted from the engine and delivered to the catalytic converter wherein it can be fired.